Molecular sieve materials, both natural and synthetic, have been demonstrated in the past to be useful as adsorbents and to have catalytic properties for various types of hydrocarbon conversion reactions. Certain molecular sieves, such as zeolites, AlPOs, and mesoporous materials, are ordered, porous crystalline materials having a definite crystalline structure as determined by X-ray diffraction (XRD). Within the crystalline molecular sieve material there are a large number of cavities which may be interconnected by a number of channels or pores. These cavities and pores are uniform in size within a specific molecular sieve material. Because the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as “molecular sieves” and are utilized in a variety of industrial processes.
Such molecular sieves, both natural and synthetic, include a wide variety of crystalline silicates. These silicates can be described as rigid three-dimensional frameworks of SiO4 tetrahedra (which have four oxygen atoms at the apexes with the silicon atom being at the center) and Periodic Table Group 13 element oxide (e.g., AlO4, BO4) tetrahedra (which have four oxygen atoms at the apexes with the Periodic Table Group 13 element being at the center). These tetrahedra are regularly and three dimensionally cross-linked by the sharing of apex oxygen atoms. This arrangement provides a three-dimensional network structure defining pores that differ in size and shape, depending on the arrangement of tetrahedral and composition of the structure. In its simplest terms, the material may be considered as a silicate material in which some of the Si4+ ions in the silicate are replaced by Periodic Table Group 13 elements such as Al3+ or B3+ ions. For each Si4+ ion replaced by a Periodic Table Group 13 element, the charge must be balanced by the inclusion in the crystal of a cation, for example a proton, an alkali metal or an alkaline earth metal cation. This can be expressed wherein the mole ratio of the Group 13 element (e.g., aluminum or boron) to the number of various cations, such as H+, Ca2+/2, Sr2+/2, Na+, K+, or Li+, is equal to unity. It is the presence of framework aluminum in aluminosilicates which is important in providing, for instance, the catalytic properties of these materials.
Molecular sieves that find application in catalysis include any of the naturally occurring or synthetic crystalline molecular sieves. Examples of these molecular sieves include large pore zeolites, intermediate pore size zeolites, and small pore zeolites. These zeolites and their isotypes are described in “Atlas of Zeolite Framework Types”, eds. Ch. Baerlocher, L. B. McCusker, D. H. Olson, Elsevier, Sixth Revised Edition, 2007, which is hereby incorporated by reference. A large pore zeolite generally has a pore size of at least about 6.0 to 7.5 Å and includes LTL, MAZ, FAU, OFF, *BEA, and MOR framework type zeolites (IUPAC Commission of Zeolite Nomenclature). Examples of large pore zeolites include mazzite, offretite, zeolite L, zeolite Y, zeolite X, omega, and beta. An intermediate pore size zeolite generally has a pore size from about 4.5 Å to less than about 7 Å and includes, for example, MFI, MEL, EUO, MTT, MFS, AEL, AFO, HEU, FER, MWW, and TON framework type zeolites (IUPAC Commission of Zeolite Nomenclature). Examples of intermediate pore size zeolites include ZSM-5, ZSM-11, ZSM-22, ZSM-57, MCM-22, silicalite 1, and silicalite 2. A small pore size zeolite has a pore size from about 3 Å to less than about 5.0 Å and includes, for example, AEI, CHA, ERI, KFI, LEV, SOD, and LTA framework type zeolites (IUPAC Commission of Zeolite Nomenclature). Examples of small pore zeolites include ZK-4, SAPO-34, SAPO-35, ZK-14, SAPO-42, ZK-21, ZK-22, ZK-5, ZK-20, zeolite A, chabazite, zeolite T, and ALPO-17.
Synthesis of molecular sieve materials typically involves the preparation of a synthesis mixture which comprises sources of all the elements present in the molecular sieve often with a source of hydroxide ion to adjust the pH. In many cases a structure directing agent is also present. Structure directing agents are compounds which are believed to promote the formation of molecular sieves and which are thought to act as templates around which certain molecular sieve structures can form and which thereby promote the formation of the desired molecular sieve. Various compounds have been used as structure directing agents including various types of quaternary ammonium cations.
The synthesis of molecular sieves is a complicated process. There are a number of variables that need to be controlled in order to optimize the synthesis in terms of purity, yield and quality of the molecular sieve produced. A particularly important variable is the choice of synthesis template (structure directing agent), which usually determines which framework type is obtained from the synthesis. This is mentioned for example in U.S. Pat. No. 4,310,440 (Wilson et al.), which teaches that “not all templating agents suitably employed in the preparation of certain species are suitable for the preparation of all members of the generic class.” It is also well known that the same template may induce the formation of different framework types. Quaternary ammonium ions are typically used as the structure directing agents in the preparation of zeolite catalysts. For example, zeolite MCM-68 may be made from quaternary ammonium ions as is described in U.S. Pat. No. 6,049,018. Other known zeolites that are typically produced using quaternary ammonium ions include SSZ-13, SSZ-15, SSZ-24, SSZ-31, and SSZ-37 as described in U.S. Pat. No. 5,281,407 and U.S. Pat. No. 5,641,393.
The “as-synthesized” molecular sieve will contain the structure directing agent in its pores, and is usually subjected to a calcination step to burn out the structure directing agent and free up the pores. For many catalytic applications, it is also desired to include metal cations such as metal cations of Groups 2 to 15 of the Periodic Table of the Elements within the molecular sieve structure. This is typically accomplished by ion exchange treatment. However, such ion exchange treatment most often does not result in full or near full exchange of the metal cations that were originally present with the desired Groups 2 to 15 metal cations.
SSZ-23 (STT framework type) has been prepared by Zones using N,N,N-trimethyladamantammonium cation as structure directing agent, as described in U.S. Pat. No. 4,859,442. It was later found that SSZ-23 could be prepared in all-silica, fluoride-mediated compositions using the same structure directing agent. SSZ-23 has also been prepared using a spiro-derivative of 2,6-dimethylpiperidine in aluminum-containing, hydroxide mediated systems (Y. Nakagawa, G. S. Lee, T. V. Harris, L. T. Yuen, S. I. Zones, Microporous and Mesoporous Materials, Vol 22, 1-3, 69-85, 1998). However, those known ammonium cation structure directing agents are relatively expensive.
The preparation of ITQ-3 (ITE framework type) is disclosed in U.S. Pat. No. 6,500,404, using N,N-dimethyl-6-azonium-1,3,3-trimethylbicyclo(3.2.1.)octane hydroxide as structure directing agent. Again, such a structure directing agent is expensive.
It is important to identify new structure directing agents and more efficient methods for the synthesis of molecular sieves to facilitate the preparation of new molecular sieves and/or to reduce the cost of making known zeolites, for instance molecular sieves of STT or ITE framework type such as SSZ-23 and ITQ-3. It is also of interest to provide a method for the preparation of molecular sieves comprising highly-dispersed metal species within their structure.